On-line instrument confirmation: how can we check that our instruments are working?

As on-line instrumentation becomes increasingly important for operating modern wastewater treatment plants it is necessary to have diagnostic methods that can indicate sensor deviation at an early stage. Examples of on-line diagnosis methods used at the Rya WWTP (Göteborg, Sweden) are presented. These include on-line mass balance calculations and soft sensors, which make indirect parameter estimates. By comparing the estimates with the corresponding on-line sensors it is possible to make control decisions and to chose alternative strategies. These comparative methods are often useful for process monitoring and diagnosis as well. Some other soft sensor applications using exponential filters are also presented.